1. Field of the Invention
The present invention relates to a solid-state image sensor, more particularly to the improvement of a solid-state image sensor that can be utilized as each pixel of a time-of-flight distance sensor that measures time-of-flight by receiving the reflection light of light irradiated on a subject using a time-of-flight measurement method (TOF: Time of flight) to measure a distance to the subject based on the time-of-flight, an image sensor that obtains the three-dimensional image of the subject, or the like, and the invention particularly relates to a solid-state image sensor in which the method of difference extraction of sorted charge in a solid-state image sensor employing a charge sorting method that can be used under unknown background light illumination.
2. Description of the Related Art
As a solid-state image sensor that can be used as a conventional time-of-flight sensor using the time-of-flight measurement method, there is known a distance image imaging device disclosed in Japanese Patent Laid-open No. 2001-281336 publication, a solid-state image sensor disclosed in Japanese Patent Laid-open No. 2003-51988 publication, a range image sensor disclosed in Japanese Patent Laid-open No. 2004-294420 publication, a time-of-flight distance sensor disclosed in Japanese Patent Laid-open No. 2005-235893 publication or the like, for example.
Herein, FIG. 1 shows an explanatory view for describing the principle of the time-of-flight measurement method. In FIG. 1, reference numeral 10 denotes an LED light source, reference numeral 12 denotes a subject on which light associated with the light emission of the LED light source 10 is irradiated, reference numeral 14 denotes a sensor where a solid-state image sensor receiving reflection light from the subject 12 is used as 1 pixel and the sensors are arranged on a two-dimensional plane in an array state.
The time-of-flight measurement method is a method in which light modulated by high frequency, e.g. 10 MHz, (high-frequency-modulated light) L1 is irradiated on the subject 12 as signal light from the LED light source 10, reflection light derived from the LED light source 10 (reflection light derived from high-frequency-modulated light) L2 is captured as a signal by the solid-state image sensors of the sensor 14, and distance from the subject 12 to the sensor 14 and the LED light source 10 is measured from phase shifting between the high-frequency-modulated light L1 and the reflection light derived from high-frequency-modulated light L2.
In such a time-of-flight measurement method, measurement is relatively easy if light that the solid-state image sensors of the sensor 14 receive is only the reflection light derived from high-frequency-modulated light L2 being reflection light derived from the LED light source 10.
However, in actual measurement sites, unknown background light L3 of various factors such as the sun, street light and room illumination exists, reflection light derived from the background light L3 (reflection light derived from background light) L4 becomes noise and received by the solid-state image sensors of the sensor 14.
FIGS. 2(a)(b) respectively show the light emission intensity of the LED light source 10 (FIG. 2(a)) and the light-receiving intensity of solid-state image sensor (FIG. 2(b)) and are timing charts showing their light emission timing and light-receiving timing, there are cases where the reflection light derived from high-frequency-modulated light L2 has extremely smaller light-receiving intensity comparing to the reflection light derived from background light L4 as shown in FIG. 2(b).
Herein, since time-of-flight FT that appears as the phase shifting between the high-frequency-modulated light L1 and the reflection light derived from high-frequency-modulated light L2 is as short as approximately 100 nsec in the distance of approximately 15 m, there has been a problem that accurate measurement could not be performed without sufficient signal-to-noise ratio in the time-of-flight measurement method.
As a method of solving the problem in the time-of-flight measurement method, various structures of a solid-state image sensor as shown in the above-described Japanese Patent Laid-open No. 2001-281336 publication, Japanese Patent Laid-open No. 2003-51988 publication, Japanese Patent Laid-open No. 2004-294420 publication or Japanese Patent Laid-open No. 2005-235893 publication, for example, have been proposed.
Herein, although the solid-state image sensor disclosed in the above-described Japanese Patent Laid-open No. 2001-281336 publication, Japanese Patent Laid-open No. 2003-51988 publication, Japanese Patent Laid-open No. 2004-294420 publication or Japanese Patent Laid-open No. 2005-235893 publication has different methods, they are basically based on a principle that two or more charge storage capacitors are connected to one photodiode, high-frequency-modulated light or pulse-emitted light is separated and stored in the above-described two or more charge storage capacitors synchronizing with its modulation or light emission time, signal-to-noise ratio is increased by reading out and averaging separated/stored charge by each given amount of time, and distance is calculated by using a signal obtained based on the separated/stored charge. It is to be noted that this principle should be appropriately called as a charge sorting method in this specification.
FIGS. 3(a) (b) (c) show an example of a light-receiving portion where charge in a solid-state image sensor is separated and stored according to the above-described charge sorting method of separating and storing charge, where FIG. 3(a) shows a circuit diagram, FIG. 3(b) shows a structural view, and FIG. 3(c) shows a potential diagram.
In FIGS. 3(a) (b), reference numeral PD denotes a photodiode, reference numeral Fd1 denotes a first storage capacitor, reference numeral Fd2 denotes a second storage capacitor, reference numeral M1 denotes a first FET switch, reference numeral M2 denotes a second FET switch, reference numeral M3′ denotes a first reset FET, reference numeral M4′ denotes a second reset FET, reference numeral Tx1 denotes a first transfer gate driving the first FET switch M1, reference numeral Tx2 denotes a second transfer gate driving the second FET switch M2, reference numeral R1 denotes the gate of the first reset FET M3′, reference numeral R2 denotes the gate of the second reset FET M4′, and reference numeral Vdd denotes a power source.
In the solid-state image sensor, the first storage capacitor Fd1 and the second storage capacitor Fd2 as a plurality of storage capacitors are connected to one photodiode PD via the first FET switch M1 and the second FET switch M2 respectively.
Therefore, by severally controlling voltage to be applied to the first transfer gate Tx1 driving the first FET switch M1 and the second transfer gate Tx2 driving the second FET switch M2, photoelectrons generated in the photodiode PD can be sorted and stored in the first storage capacitor Fd1 and the second storage capacitor Fd2.
Herein, the first reset FET M3′ is used for initializing charge stored in the first storage capacitor Fd1, and the second reset FETM4′ is used for initializing charge stored in the second storage capacitor Fd2, and when voltage is applied severally to the gate R1 and the gate R2, the voltage of the storage capacitor Fd1 and the storage capacitor Fd2 is initialized to the voltage of the power source Vdd.
When light is irradiated on the potential in the initial state after the above-described reset (refer to FIG. c-1), photoelectrons are generated in the photodiode PD.
After that, when voltage is applied to the first transfer gate Tx1, the photoelectrons generated in the photodiode PD run down to the first storage capacitor Fd1 side according to the potential (refer to FIG. c-2).
Herein, when the voltage of the first transfer gate Tx1 is returned and voltage is applied to the second transfer gate Tx2 on the contrary, the photoelectrons generated in the photodiode PD run down to the second storage capacitor Fd2 side according to the potential (refer to FIG. c-3).
When the above-described application of voltage to the first transfer gate Tx1 and the second transfer gate Tx2 is repeated at high frequency, each temporarily sorted charge is severally stored in the first storage capacitor Fd1 and the second storage capacitor Fd2 as shown in FIG. c-4.
After the charge stored in the first storage capacitor Fd1 and the second storage capacitor Fd2 are read out in this manner by a read out circuit (not shown), voltage is applied to the first transfer gate Tx1 and the second transfer gate Tx2 to reset (FIG. c-5), and the charge is initialized.
Therefore, according to the solid-state image sensor shown in FIGS. 3(a) (b) (c), by reading out and averaging the charge stored in the first storage capacitor Fd1 and the second storage capacitor Fd2 by each given amount of time, the signal-to-noise ratio is increased and distance can be calculated by using a signal obtained based on the separated/stored charge.
Incidentally, in the case where the solid-state image sensor having the constitution shown in FIGS. 3(a) (b) (c) is used as the solid-state image sensors of the sensor 14 and the light as shown in FIGS. 2(a) (b) is measured in the constitution shown in FIG. 1, charge as shown in FIG. 4, for example, is stored in the first storage capacitor Fd1 and the second storage capacitor Fd2 of the solid-state image sensor after passing the given time.
In this embodiment, the photodiode is expediently shown as PN junction type in the drawings. However, the transfer efficiency and the transfer rate between the photodiode and the transfer gate will be able to be improved by using an implanted photodiode and/or photo gate type detector.
Meanwhile, in FIG. 4, reference numeral 42 expediently denotes a component by the reflection light derived from high-frequency-modulated light L2 synchronized with the first transfer gate Tx1, which is derived from the LED light source 10, reference numeral 45 expediently denotes a component by the reflection light derived from high-frequency-modulated light L2 synchronized with the second transfer gate Tx2, which is derived from the LED light source 10, reference numeral 43 expediently denotes a component by the reflection light derived from background light L4 derived from the background light L3, which is synchronized with the first transfer gate Tx1, and reference numeral 44 expediently denotes a component by the reflection light derived from background light L4 derived from the background light L3, which is synchronized with the second transfer gate Tx2, but actually, the component of reference numeral 42 and the component of reference numeral 43 cannot be separated by each component, and the component of reference numeral 44 and the component of reference numeral 45 cannot be separated by each component as well.
Then, in the case where the contribution of the background light L3 is large in the constitution shown in FIG. 1, there has been a problem that charge severally stored in the first storage capacitor Fd1 and the second storage capacitor Fd2 was saturated as shown in FIG. 5, sorting action of charge became invalid.
Specifically, in FIG. 5, reference numeral 52 and reference numeral 53 severally express saturated charge, and reference numeral 54 express charge overflown from the first storage capacitor Fd1 and the second storage capacitor Fd2.
In this case, charge exceeded a potential difference between the photodiode PD and the first storage capacitor Fd1 controlled by the first transfer gate Tx1, and a potential difference between the photodiode PD and the second storage capacitor Fd2 controlled by the second transfer gate Tx2, and sorting action of charge became invalid.
Conventionally, to avoid malfunction caused by such saturation of charge, device such that near infrared light has been used as the LED light source 10, a bandpass filter having the wavelength of the light has been installed accordingly to the solid-state image sensors of the sensor 14, to attenuate as much reflection light derived from background light L4 derived from the background light L3 as possible.
However, since sunlight contains much near infrared light, it has been difficult to eliminate its influence considering the use in outdoor.
Further, the invention described in the above-described Japanese Patent Laid-open No. 2004-294420 publication discloses a device such that influence of background light is reduced by separately observing background light before the emission of modulated light.
However, to realize an essentially high-sensitivity sensor, a measure for solving such a problem of saturation caused by background light is necessary, because the smaller capacitance of the first storage capacitor Fd1 and the second storage capacitor Fd2 is desirable for higher sensitivity, but in this case, there is a fear that charge amount reaching saturation becomes smaller and the capacitors cannot be used by being saturate by the background light.